1. Field of the Invention
The present invention relates generally to construction of multi-layer thermoplastic laminates having microstructures on one surface thereof and, more particularly, to multi-layer thermoplastic laminates of dissimilar polymeric films having microstructures on one surface thereof suitable, for example, for use as retroreflective sheeting.
2. Description of the Related Art
Thermoplastic sheeting products having a plurality of microstructures on one surface thereof have many commercial applications. Different types of microstructures can be provided on the sheeting depending on the purpose of the ultimate product. For example, it is known to provide on one surface of a thermoplastic film a plurality of microprismatic type channels in a pre-determined pattern. The channels can contain a deposit of a suitable chemical composition, for example, that changes color in the presence of a bodily fluid drawn into the microprismatic type channels by capillary action. Such products can be used in medical diagnostic devices, such as home pregnancy test kits. Another example of a commercial application for thermoplastic films having microprismatic structures is in fuel cells in accordance with well-known electro-chemical technology.
Another commercially significant application of thermoplastic films having microstructures on one surface is for use as retroreflective sheeting for highway signs and the like. In this type of application, the microstructures on one surface of the film are cube-corner retroreflective elements. The microstructures are in the form of an array of several xe2x80x9ccubexe2x80x9d elements each consisting of three essential mutually perpendicular faces which serve to receive incident light and retroreflect the light through 180xc2x0 approximately parallel to its incident path and back to its source. The term xe2x80x9ccube-cornerxe2x80x9d has long been recognized in the art to refer to essentially any structure of three mutually perpendicular faces without regard to the size or shape of each face or the optical axis of the element so provided. An early example of a macro-sized cube-corner type reflector is disclosed in Stimsonite, U.S. Pat. No. 1,906,655, issued May 2, 1933. Another example of a macro-sized cube-corner type reflector is in Heenan, U.S. Pat. No. 3,332,327 issued Jul. 25, 1967, which teaches a pavement marker construction.
Unlike a typical pavement marker construction, in which the cube corner elements may be relatively large in size, in reflective sheeting of the microprismatic type the cube-corner elements are reduced in size to be useable on a relatively thin film substrate. Microprismatic retroreflective sheeting is particularly useful in the construction of highway signage, for example, in which an aluminum sign blank is covered with a layer of light reflective sheeting bearing suitable indicia for informing drivers of a particular highway condition. Microprismatic retroreflective sheeting has also found applications in clothing and for vehicle graphics.
Methods to form thin film materials with microprismatic retroreflective elements on one surface thereof include embossing and casting. In many instances, the sheeting comprises a transparent polymethyl methacrylate (i.e. acrylic) substrate, or film, although various other forms of thermoplastic material may be used such as polycarbonate, polyvinyl chloride, polyolefin or polyurethane, for example.
An example of a highly efficient method and apparatus for continuous embossing of a resinous film with cube-corner retroreflective elements is disclosed in Pricone et al., U.S. Pat. No. 4,601,861, the disclosure of which is incorporated herein by reference in its entirety. In this process, a continuous web of transparent film is fed through an embossing machine along with a carrier film in which the transparent film is heated above its glass transition temperature and compressed against an embossing tool such that resinous film material flows into the pattern of the tool. The film laminate is then cooled, and stripped from the tool. The tool may be constructed by a process of the type disclosed in Montalbano, U.S. Pat. No. 4,460,449, the disclosure of which is incorporated herein by reference in its entirety. The tool of this patent is capable of creating very small, accurately formed microcube-corner elements on the order of several thousand per square inch of film. The ""861 patent discloses that a second layer of thermoplastic material, such as a material containing UV inhibitors, can be simultaneously run through the embossing equipment with the first transparent film and the carrier, either by use of an additional feed roller or by prelaminating to the first transparent film. The ""861 patent gives as an example a first transparent film of a rubber modified polymethyl methacrylate, and an additional layer of an acrylic material having significant UV inhibitors.
The above-described process has proved to be quite commercially successful in producing microprismatic sheeting having considerable retroreflective brilliance, particularly when used with single layer polymer films, or multi-layer films in which the polymeric materials have similar physical properties and substantially equivalent refractive indexes.
It also would be desirable to have a process for producing embossed multi-layer film laminates comprised of dissimilar polymeric film materials, that is, film materials having different physical properties and/or refractive indexes that are not substantially equivalent. Many modern highway signage applications now require extended outdoor durability of up to 12 years. The polymers selected to manufacture micro-prismatic retroreflective sheeting must be able to withstand extended sun light exposure and other harsh environmental conditions. Unfortunately, many polymers which have desirable properties for retroreflective microprismatic sheeting are not very weatherable. For example, polycarbonate is a desirable polymer to create cube corner microprisms because it has a refractive index of approximately 1.58. Those familiar with the art of optics and retroreflective structures will realize that such a high refractive index is desirable because it can yield retroreflective performance at wide angles of incident light. However, polycarbonate is an inherently poor weathering polymer. Within one year of outdoor exposure, it will yellow, crack and become hazy. While the use of ultra-violet absorbers with the polycarbonate film can extend its exterior performance, to obtain adequate durability for an outdoor highway signage application, an ultra-violet light screening layer or cap-layer must be placed on top of the polycarbonate. In order to provide adequate protection, the cap-layer film itself should be a polymer with exceptional durability properties such as polymethyl methacrylate or copolymers thereof. Accordingly, one reason that multi-layered film structures of dissimilar polymeric films are desirable is so that an ultra-violet light screening layer or cap-layer can be utilized to enhance the durability of an otherwise unweatherable polymer.
Another reason it is desirable to have a multi-layer laminate structure is to create sheeting materials with enhanced retro-reflective performance that also have improved flexibility. Generally speaking, a film to be embossed with cube-corner retroreflective elements should be somewhat tough and rigid, such as acrylic or polycarbonate. This is so because in order for the finished product to maintain its reflective brilliance, the cube-corner elements must be relatively rigid to retain their geometric shapes. Virtually any slight distortion of the rigid cube wall angles affects retroreflectivity. However, rigid polymers in themselves are not considered advantageous when used alone in applications that require greater flexibility, such as clothing. Although sheeting of such rigid polymers is capable of being wound on a roll during and after manufacturing, it is not sufficiently flexible to allow it to be sharply bent or folded. If the sheeting is sharply bent, it will be subject to crazing, which degrades retroreflective performance. Conversely, if the retroreflective film is entirely comprised of a soft, flexible polymeric film such as plasticized polyvinyl chloride, after embossing the corner cube retroreflective elements will not fully retain their ideal geometric configuration. When subjected to tension or pressure, the microprisms will become distorted. Accordingly, it is known to construct multi-layer film laminates wherein a base layer of a relatively rigid polymer is provided with cube-corner retroreflective elements and that layer is bonded to a more flexible polymer layer, such as polyurethane or flexible polyvinyl chloride. The latter serves to add body to the retroreflective layer such that the resultant laminate is more suitable for use in applications requiring flexibility. Such structures are disclosed, for example, in prior art U.S. Pat. Nos. 3,684,348; 3,689,346; 3,810,804; 3,811,983; and 3,935,359, of which U.S. Pat. Nos. 3,684,348 and 3,689,346 describe a lack of success in the manufacture of cube corner formations by embossing. U.S. Pat. No. 5,450,235 also discloses multi-layer cube corner sheets made by a casting technique.
A difficulty that has been encountered in producing multi-layer embossed retroreflective sheeting is that if the polymer layers have different refractive indexes, a hazy appearance may result during embossing which causes loss in retroreflectivity values. Applicants"" assignee has found that essentially this phenomenon results when the interface between polymer layers of differing refractive indexes is not optically smooth; this causes light to be refracted in undesirable directions across the interface. Embossing the retroreflective elements in the base film layer of a multi-layer structure can cause the interface between the layers to develop a rough or random geometry, resulting in a non-optically smooth interface. The extent of this effect may depend on the thickness of the base film and the depth of the micro prismatic pattern embossed therein.
A second problem concerns heat degradation of one of the polymer films. If two polymers with significantly different glass-transition temperatures are combined in a multi-layer structure, the elevated temperatures required to emboss the base layer may initiate heat degradation of the top layer. This is particularly true when a flexible polymer top layer and a rigid polymer base layer are embossed together. For example, if flexible polyvinyl chloride is laminated over polycarbonate, the higher temperatures required to emboss retroreflective elements in the polycarbonate base layer will begin heat degradation of the polyvinyl chloride top layer.
Accordingly, it is desirable to provide a novel method for forming multi-layer embossed microprismatic sheeting where the layers are comprised of dissimilar polymeric materials, that is, polymeric materials that have different physical properties and/or refractive indexes that are not substantially equivalent. It is further desirable to provide such sheeting wherein the microprismatic structures are cube corner retroreflective elements and wherein the interface between the dissimilar polymeric materials is smooth and not random such that light will not be refracted in undesirable directions across the interface even if the refractive indexes of the polymeric materials are not substantially equivalent. Still further it is desirable to provide a method to produce such sheeting wherein thermal degradation of one of the polymeric films will not be induced by the heat used to emboss another polymeric film.
The present invention improves over the prior art by providing a method of continuously forming a multi-layer laminate of thermoplastic polymeric films with one surface thereof having a precision pattern of microprismatic elements embossed thereon. Preferably, the polymeric films have different physical properties, so that polymeric materials can be selected to optimize the performance of the final embossed product. Generally, such polymeric films also will have indexes of refraction that are not substantially equivalent. The method is performed with the use of a generally cylindrical endless metal embossing tool with an outer surface which is formed with the reverse of the pattern to be formed on the surface of the sheeting. The laminate is formed by continuously feeding onto a heated embossing tool a superimposed first resinous film and a first carrier film wherein the first resinous film is pressed against the embossing tool and is heated above its glass transition temperature thereby becoming embossed with the pattern, while the first carrier film remains at a temperature below its glass transition temperature. The first carrier film is then removed from superimposed relation with respect to the first resinous film. Next, a second resinous film and second carrier film are superimposed on the unembossed surface of the first resinous film and are heated such that the two resinous films become bonded together. The resulting laminate is then cooled and is stripped from the embossing tool. If the first and second resinous films have refractive indexes that are not substantially equivalent, then preferably the surfaces of the first and second resinous films that are in contact with one another are each optically smooth. Thus, a smooth interface is formed between the two resinous films, resulting in an embossed multi-layer product having exceptional retroreflective capability when used as retroreflective sheeting. The first and second resinous films may be selected for other performance characteristics when the laminate is to be used in applications other than retroreflective sheeting.